The present invention relates to a network system composed of a plurality of devices connected to each other by logical paths constructed on a bidirectional data bus for transferring data among the plurality of the devices.
Conventionally, a multiple of electronic devices are interconnected to build up a network system. In such a network system, the devices are physically coupled to each other by a number of cables which occupy considerable space. Once the cables are disconnected from the devices, it is laborious to restore the connection of the network system. In order to obviate this inconvenience, the devices are physically connected to each other by a common cable.
A plurality of logical paths are formed in the physical common cable to transfer data among the electronic devices in a sophisticated network system. FIG. 1 shows an example of such a network system. In this example, the network system includes seven numbers of electronic devices which are denoted by nodes N1-N7. The nodes N1-N7 are physically interconnected to each other by means of cables according to predetermined sequence.
Each of the nodes N1-N7 is constructed as shown in FIG. 2. The node is equipped with a physical layer 11 having input/output terminals 19-1, 19-2 and 19-3. One cable is connected to one of the input/output terminals 19-1, 19-2 and 19-3. The physical layer 11 operates to transmit and receive data. A link layer 12 is disposed over the physical layer 11. Though not shown in the figure, higher layers are further disposed on the link layer 12.
In the network system constructed as described above, for example, the node N2 may transfer data to the nodes N4 and N5. In this case, the node N2 transfers data to the node N5 with a transfer time lag d25. The node N2 also transfers data to the node N4 with another transfer time lag d24. The former transfer lag d25 may be longer than the latter transfer time lag d24 depending on distances of the nodes N4 and N5 from the node N2. Namely, the data transmitted from the node N2 does not arrive at the nodes N4 and N5 at the same time, but is received by the nodes N4 and N5 at different times.
Normally, such a transfer time lag varies dependently on a distance between the nodes. The transfer time lag becomes maximum when data is transferred from one extreme node to another extreme node in the network system. In a music network system, one node may transmit audio data on realtime basis to a plurality of nodes so as to control the same to generate musical sounds according to the audio data received by the plurality of the nodes. However, in reality, the plurality of the nodes could not concurrently generate the musical sounds due to difference of the transfer time lags.
In order to obviate such a drawback, each of the nodes N1-N7 operates to compensate for the relative difference in the data transfer time lag according to an absolute reference time which is predetermined to cover the maximum or worst transfer time lag in the data communication of the existing network system. Namely, the node receiving the data operates to temporarily withhold or retain the received data in the link layer 12 before the received data is passed to the higher layer for an adjustive time interval which is determined by subtracting the transfer time lag attributive to the receiving node from the absolute reference time. In this case, the receiving node is provided with a static data buffer in the link layer 12 for temporarily withholding or retaining the received data for the predetermined adjusting time interval. By such a manner, the network system can assure concurrent or synchronous treatment of realtime data received by the respective nodes.
However, the absolute reference time is fixed and cannot be set shorter than the maximum or worst transfer time lag in the conventional network system. Therefore, the received data may be unnecessarily or unduly withheld or retained in the link layer when the data is transferred between near nodes with a short transfer time lag relative to the reference time. This would unduly limit a nominal data transfer rate, thereby disadvantageously hindering communication performance of the nodes. Further, each node must be equipped with the buffer having an unduly great size.
In the music network system, the node normally applies signal processing such as digital/analog conversion to the received data passed from the link layer 12 so as to generate the musical sound. The node may consume a substantial time for the signal processing. Such an operational time lag may vary dependently on different functions of the respective nodes. Therefore, final acoustic output timings of the realtime audio data are made inconsistent among the plurality of the nodes due to the difference of the operational time lags of the respective nodes even though the transfer time lags are compensated throughout the network system.